Occasion sometimes arises to interdict the movement of personnel in an outdoor area, or to monitor an area to give warning of the presence of intruders therein. Systems detecting the presence of intruders by seismic means are known. Aune et al. U.S. Pat. No. 3,585,581 and Barowitz et al. U.S. Pat. No. 4,090,180 are examples of such systems.
Those patents rely on "propagating" or Rayleigh seismic waves, in the frequency range of 30 to 100 Hertz. A feature of such waves is that their amplitude characteristics are very dependent on the nature of the soil through which they must travel, so that seismic apparatus adjusted to operate with satisfaction in one particular soil environment may require readjustment for satisfactory operation in a different soil environment.